1. Field of the Invention
The present invention relates to a printing head capable of ejecting a liquid such as an ink, an ink jet printing apparatus that prints an image using the printing head, and an ink jet printing method.
2. Description of the Related Art
In an ink jet printing apparatus, a printing head is used which is capable of ejecting a liquid such as an ink by using an electro-thermal converter (heater) or a piezo-element. As shown in FIG. 9A, in a printing head H using an electro-thermal converter 101, a liquid in a flow path 102 is foamed by heat of the electro-thermal converter 101 (see FIGS. 9B and 9C), and by utilizing the foam energy of air bubbles B generated at this time, the liquid can be ejected from an ejection port 103. The air bubbles B defoam as shown in FIGS. 9D and 9E. To the head H of the present example, a movable valve 104 is provided in the flow path 102 in order to effectively cause the foam energy of the air bubbles B to act in the direction of the ejection port 103. An ink jet printing apparatus using such a print head H is capable of printing an image on a printing medium by applying the liquid ejected from the ejection port 103. In the printing apparatus, demand for higher speed printing has been increased.
For such a printing head H, a new problem has become apparent as the printing speed increases. As shown in FIG. 9D, when dividing a liquid column pushed out from the ejecting port 103 to form a droplet (main-droplet), a sub-droplet Ds referred to as a satellite is also formed along with main-droplet Dm as shown in FIG. 9E. When these main-droplet Dm and sub-droplet Ds have landed on the printing medium deviated from each other, the image quality of the printed image may deteriorate. As shown in FIG. 10A, the ejecting timing of the sub-droplet Ds is later than that of the main-droplet Dm, and the ejecting speed Vs of the sub-droplet D is lower than the ejecting speed Vm of the main-droplet Dm. Therefore, as the relative moving speed Vf of the head H and the printing medium W becomes higher, deviation d of the landing positions of the main-droplet Dm and the sub-droplet Ds becomes larger (see FIGS. 10B and 10C). FIGS. 10A, 10B, and 10C illustrate that the printing medium W moves against the head H. D1 is a dot formed on the printing medium W by a main-droplet Dm, and D2 is a dot formed on the printing medium W by a sub-droplet Ds.
Conventionally, in order to keep the deviation of the landing positions of the main-droplet and the sub-droplet small, a distance h (see FIG. 10A) between an ejection port face (face where the ejection port is located) of the printing head and the printing medium is narrowed, or the ejecting speed of a liquid is increased.
Meanwhile, Japanese Patent Laid-Open No. 2000-263788 describes a configuration for matching the ejecting directions of main-droplet and sub-droplet of ink. When a nozzle portion including an ejection port and a flow path is formed of a plurality of materials, a difference in surface energy among the materials, in other words, a difference in wettability to the ink, occurs. The configuration described in Japanese Patent Laid-Open No. 2000-263788 is provided focusing on the fact that the deviation in the ejecting directions of the main-droplet and the satellite occurs due to such difference in wettability to the ink. That is, the ejection port face is inclined so that the part of the flow path on the side where a material with less surface energy is located is made shorter than the part of the flow path on the side where a material with more surface energy is located. This causes the ejecting directions of the main-droplet and the satellite to be made coincident.
However, when attempting to shorten the distance h (see FIG. 10A) between the ejection port face of the head and the printing medium in order to keep the deviation of landing positions of the main-droplet and the sub-droplet small, there is a limit to shortening the distance h. When the distance h is too short, the printing medium may contact the ejection port face of the head as a result of cockling on the printing medium where a liquid is provided. In addition, poor liquid ejection may also occur as a result of a liquid bounced back from the surface of the printing medium or a liquid in mist form attaching to the ejection port face. When attempting to increase the ejecting speed of liquid in order to keep the deviation of landing positions of the main-droplet and the sub-droplet small, there also is a limit to speeding up.
Thus, keeping small the deviation of landing positions of the main-droplet and the sub-droplet while achieving higher printing speed is difficult just by shortening the distance h between the ejection port face of the head and the printing medium, or by speeding up the ejecting speed of liquid.
On the other hand, Japanese Patent Laid-Open No. 2000-263788 only discloses a configuration for matching the ejecting directions of the main-droplet and the sub-droplet as shown in FIG. 10A. With such a configuration, solving the problem associated with the increase in the printing speed as shown in FIGS. 10B and 10C, i.e., suppressing the increase in deviation of the landing positions of the main-droplet and the sub-droplet, cannot be achieved.
Conventionally, as described, the deviation of the landing positions of the main-droplet and the sub-droplet that increased along with the increase in printing speed could not be sufficiently suppressed. In particular, complying with a request desired for an ink jet printing apparatus for industrial use was difficult, i.e., a request for higher printing speed and higher quality of printed image. In an ink jet printing apparatus for industrial use, for example, when printing with barcodes, the deviation of landing positions of the main-droplet and the sub-droplet will be critical. Barcodes are printed information made of combinations of black bars and white spaces different in thickness. Thus, when the deviation of the landing positions of the main-droplet and the satellite increased, sizes or positions of the bars or spaces move out of readable standards, which may make the barcodes unable to be read.
FIGS. 11, 12A, and 12B are explanatory views of printing results in the case of landing positions of the main-droplet and the sub-droplet deviated in so-called serial scan type and full line type ink jet printing apparatuses.
In the so-called serial scan type ink jet printing apparatus, as shown in FIG. 11, an image is sequentially printed on the printing medium W by repeating an operation of ejecting a liquid while the head H moves in the main scanning direction of an arrow X and an operation of conveying the printing medium W in the sub-scanning direction of an arrow Y. The printing method in FIG. 11 is a bi-directional printing method that prints the image when the head H moves both in the forward direction of an arrow X1 and in the backward direction of an arrow X2. Upon the former forward scanning, a dot D2 is formed deviated from the center of a dot D1 in the traveling direction (X1 direction) of the head H. On the other hand, upon the latter backward scanning, the dot D2 is formed deviated from the center of the dot D1 in the traveling direction (X2 direction) of the head H. When the scanning speed (moving speed in the arrows X1 and X2 directions) of the head H is relatively low, the dot D2 is formed within the dot D1 as shown in FIG. 11. However, when the scanning speed becomes high, the dot D2 is formed outside the dot D1. As a result, when the barcodes are printed at high speed, the barcodes may be unable to be read.
In the so-called full line type ink jet printing apparatus, as shown in FIG. 12A, an image is continuously printed on the printing medium W by ejecting a liquid from the head H while continuously conveying the printing medium W in the arrow Y1 direction with the head H being fixed. The dot D2 is formed deviated from the center of the D1 in the direction opposite (arrow Y2 direction) the conveying direction (arrow Y1) of the printing medium W. The arrow Y2 direction is a relative moving direction of the head H against the printing medium W. When the conveying speed of the printing medium W is relatively low, the dot D2 is formed within the D1 as shown in FIG. 12A. However, when the conveying speed of the printing medium W is high, the dot D2 is formed outside the dot D1 as shown in FIG. 12B. As a result, when the barcodes are printed at high speed, the barcodes may be unable to be read.